1. Field of the Invention
This invention relates to sound and more particularly to the suppression of noise generated by equipment such as, engines, generators, fans, compressors, etc.
2. Description of the Prior Art
It is well known that engines, compressors, fans, etc. can generate such noise that normal conversation near them is essentially impossible. Furthermore, such loud noise can impair hearing and cause phychological effects that reduce the efficiency of a workman. As a result of the findings from research in this area of technology, great interest is now being shown in various techniques for regulating sound emissions from a wide variety of operating equipment. Since man would benefit from the noise reduction of operating equipment, a measure of sound, weighted to the response of the human ear, was developed. The weighted scale is referred to as the "A" scale. The noise level is measured in decibels, weighted to the A scale and called dBA. Various experiments have been performed to determine the dBA values for different noise conditions. It was found that 130 dBA was the threshold of pain. A value of 50 dBA was found to be the average level for office conversation. Since it was necessary to drop the sound level of field equipment down to 50 dBA, the Environmental Protection Agency issued a temporary goal of 75 dBA for field equipment with an ultimate goal of 65 dBA. It is to the 65 dBA level and lower that this invention disclosure is directed.
Several journals have published the results of acoustical suppression of operating engines. These results used the standard techniques of employing acoustical materials, of designing special mufflers, of designing silencers, and of using various vibration isolators. A large industry has developed around sound suppression, providing new materials and test data on the materials. A review of the literature shows that the materials and techniques of sound suppression center mainly around buildings and operating equipment found in buildings. It is relatively easy to suppress noise sources in a building because there is not a weight or volume restriction in general. In fact, one could build a room around a noisy engine if need be. The suppression problem is more severe for those noise sources that are in the field, such as an engine-generator set or compressor.
One of the major difficulties in acoustically suppressing an operating engine is a resultant engine overheating problem. If an engine is completely encased so that no noise can escape, the engine can overheat easily. If holes are made for engine intake air and exhaust air, then sound can escape through these holes. Designers have used mufflers, silencers, curved ducts, new materials and a variety of other techniques to suppress some engines. Published results show that these techniques have suppressed some engines (air compressors) down to 72 dBA. Significantly lower values have not been published to the best of this inventor's knowledge. The military has developed sound suppressors for military engine-generator sets, but at the expense of heating up the surface of the sound suppressor. For reasons other than sound, a hot surface surrounding an engine-generator set is not advantageous from a military viewpoint as it can easily be detected by state-of-the-art thermal viewers.